1. Field of the Invention
This invention relates to an anti-skid brake control method for vehicle of X-brake conduit system.
2. Description of the Prior Art
U.S. Pat. No. 4,740,040 discloses:
An anti-skid control vehicle braking apparatus with pairs of front and rear wheels; wheel speed sensors associated with the wheels; first and second fluid pressure control valve devices for respectively controlling the brake fluid pressure of the wheel cylinders of the front wheels; a control unit receiving outputs of the wheel speed sensors for measuring or judging the skid conditions of the front and rear wheels and for generating instructions for controlling the fluid pressure control valve devices; a valve apparatus for generating a fluid pressure in accordance with the lower one of the brake fluid pressures of the front wheels controlled with the control valve devices, being arranged between the front wheel cylinders and rear wheel cylinders. The control unit discriminates the frictionally lower one of the sides of the road on which the wheels are running, on the basis of the measuring or judging results of the skid conditions of rear wheels, combines logically the measuring or judging results of the skid conditions of the rear wheels with the measuring or judging result of the skid condition of one front wheel running on the lower side for generating the instruction for controlling one of fluid pressure control valve devices, and generates the instruction for controlling the other control valve device, on the basis of the measuring or judging result of the skid condition of the other front wheel running on the frictionally higher side independently of those of the rear wheels.
The above described fluid-pressure control apparatus for 2-channel anti-skid apparatus can be smaller, lighter and more inexpensive than the prior fluid-pressure control apparatus. Even when the rear wheel tends to lock sooner than the front wheel, the rear wheel can be surely prevented from locking, by the above fluid-pressure control apparatus. Further, all of the wheels can be securely prevented from locking thereby. The steering and the directional stability of the vehicle can be better. The above effects of the lock-prevention of the rear wheel and of the security of the directional stability are caused not only by the control method of the above fluid-pressure control apparatus, but also greatly by the valve apparatus or the pressure-selecting apparatus arranged in the above fluid pressure control apparatus.
However, it is required that a fluid pressure control apparatus is further smaller, lighter and more inexpensive, in order that an anti-skid apparatus is widely used in the world.
For example, the Japanese Patent Opening Gazette No. 160342/1986 discloses a fluid pressure control apparatus for 2-channel X-brake conduit type anti-skid apparatus in which the above-described pressure selecting apparatus is not provided. In the control method of the above fluid pressure control apparatus, when it is detected that any of the front wheels almost locks, the fluid pressure is decreased in the one conduit system to which the almost locking front wheel belongs. And when it is detected that any of the rear wheels almost locks, the one rear wheel which tends to lock sooner than the other rear wheel is made to remain locked. When also the other rear wheel almost locks, the brake fluid pressure is decreased in the one conduit system to which the other rear wheel belongs.
However, in the above-described fluid pressure control apparatus, both of the rear wheels sometimes almost lock, or one of the rear wheels remains locked, when the other of the rear wheels almost locks. When the vehicle with the above-described fluid pressure control apparatus is running on a road which has such a braking frictional coefficient as to be intermediate (intermediate-.mu.) or low (low-.mu.), the one rear wheel sometimes continues to lock. At that time, the directional stability of the vehicle is deteriorated much. Even when the vehicle is running on a road which has a high braking frictional coefficient (H-.mu. road), the one rear wheel sometimes continues to lock. At that time, the so-called "flat-spot" phenomenon (partial wearing) occurs on the tyre of the one rear wheel which continues to lock. At an extreme case, there is the danger that the one rear wheel bursts.